Nonlinear distortion compensation methods for microwave power amplifiers (power amplifiers hereafter) include a predistortion method. The predistortion method uses a predistorter to add, to a power amplifier input signal beforehand, a distortion compensation component for cancelling out a distortion component generated by the power amplifier.
The power amplifier generally provides high efficiency when it is operated at around the saturation output power. As the power amplifier approaches its saturation output power, the intermodulation distortion (distortion component hereafter) increases because of the nonlinear characteristics. In addition, the distortion component has frequency dependent. Predistorters that can compensate for the frequency dependent distortion component include a power series digital predistorter (digital predistorter hereafter) that compensates for the frequency dependence of the distortion component (refer to S. Mizuta, Y. Suzuki, S. Narahashi, and Y. Yamao, “A New Adjustment Method for the Frequency-Dependent IMD Compensator of the Digital Predistortion Linearizer,” IEEE Radio and Wireless Symposium 2006, pp. 255-258, January 2006.).
FIG. 1 shows a conventional structure of a digital predistorter 900 and its peripheral equipment. In the example shown, a digital input transmission signal includes an I-phase signal and a Q-phase signal (I/Q signals hereafter). The digital predistorter 900 includes a linear transfer path 901A, a distortion generation path 901B, dividers 902, combiners 903, digital-to-analog converters (DACs) 904, analog-to-digital converters (ADCs) 905, an distortion observer 906, and a controller 907. The linear transfer path 901A includes a delay unit 901A1. The distortion generation path 901B includes a distortion generator 901B1, a distortion vector adjuster 901B2, and a frequency characteristic compensator 901B3. The dividers 902 divide the input transmission signal into the linear transfer path 901A and the distortion generation path 901B. The combiners 903 combine the outputs of the linear transfer path 901A and the outputs of the distortion generation path 901B. The digital-to-analog converters (DACs) 904 convert the outputs of the combiners 903 (digital I/Q signals with distortion compensation components added thereto) to analog I/Q signals. The analog-to-digital converters (ADCs) 905 convert the outputs (analog I/Q signals) of a feedback signal generator 960 that takes in a part of the output of an amplifier 950 as a feedback signal to digital I/Q signals. The distortion observer 906 detects a distortion component from the outputs of the ADCs 905. The controller 907 adjusts vector coefficients (amplitude and phase) to be set in the distortion vector adjuster 901B2 and one or more frequency characteristic compensator coefficients (amplitude and phase) to be set in the frequency characteristic compensator 901B3, in accordance with the output of the distortion observer 906.
The amplifier 950 includes a quadrature modulator 951 for performing quadrature modulation of the analog I/Q signals output from the digital predistorter 900, a frequency upconverter 952 for converting the frequency of the modulated output to the carrier frequency, and a power amplifier 953 for performing power amplification of the frequency-converted signal, and supplies the power-amplified signal from an output terminal 970 to an antenna, for example, via a duplexer, not shown.
The feedback signal generator 960 includes a coupler 961 that extracts a part of the output of the amplifier 950 as a feedback signal, a frequency downconverter 962 for converting the frequency of the feedback signal, and a quadrature demodulator 963 for performing quadrature demodulation of the down-converted feedback signal.
FIG. 2 shows an example structure of the frequency characteristic compensator 901B3. The frequency characteristic compensator 901B3 includes a J-point FFT 901B31, a complex multiplier 901B32, and a J-point IFFT 901B33. The FFT 901B31 converts the input signal of the frequency characteristic compensator 901B3 into the frequency domain. The complex multiplier 901B32 multiplies each of M bands formed by dividing the upper band and lower band of the distortion component, as shown in FIG. 3, by a frequency characteristic compensator coefficient given by the controller 907 (phase and amplitude adjustment). The output of the FFT 901B31 outside the divided bands is directly input to the IFFT 901B33, which is not shown in the figure. The IFFT 901B33 converts the output of the complex multiplier 901B32 into the time domain.
FIG. 4 shows a flowchart illustrating processing for frequency characteristic compensator coefficients that minimize the distortion component, in the frequency characteristic compensator. The controller 907 specifies one band where the frequency characteristic compensator coefficient is to be adjusted (band specification step S900), adjusts the frequency characteristic compensator coefficient to reduce the power of the distortion component in the specified band to the minimum level (or a target value), and sets the coefficient in the frequency characteristic compensator (frequency characteristic compensator coefficient adjustment steps S901, S902). When the distortion component power in the specified band is minimized (condition 1) and when the distortion component power in each of all bands is minimized (condition 2), the controller 907 ends the frequency characteristic compensator coefficient adjustment process and sets the obtained frequency characteristic compensator coefficients in the frequency characteristic compensator (S903). If the conditions 1 and 2 are not satisfied, the controller 907 goes back to the band specification step and repeats the steps S900, S901, and S902 until the conditions 1 and 2 are satisfied (S903). By adjusting the frequency characteristic compensator coefficients in all bands in accordance with the processing as described above, the frequency-dependent distortion component is compensated for.